Method of producing fire resistant lignocellulose hardboard products



aired rates METHQD F PRGDUCING FIRE RESISTANT LIGNUCELLULOSE HARDBOARDPRODUCTS No Drawing. Application April 13, 1955, Serifl No. 501,194

7 Claims. (Cl. 117-103) The present invention relates to the preparationof lignocellulose hardboard products characterized by greatly improvedresistance to fire. More particularly, the invention relates to a novelmethod of impregnating hardboard sheets with water soluble phosphates inamounts sufiicient to render the sheets fire retardant.

It has long been recognized by the hardboard, tileboard, and buildingindustries that the production of fire retardant hardboard productswould greatly enlarge their fields of usefulness. To this end, manyproposals have been made to incorporate in hardboard products variousfire proofing agents. It has been proposed, for example, to incorporatein these products such materials as borax, boric acid, nitrogencontaining compounds, chlorinated hydrocarbons, and the like agents. Thecompounds have been added during the production of the hardboard and byimpregnation of the finished product. However, the prior art methodshave uniformly failed to accomplish the desired result. The treatedhardboard has been subject to bleeding of the agents, crystallizationthereof, and the like unsatisfactory properties. Above all, it hasheretofore been almost impossible to obtain sufiicient retention of theadded agents to impart any great degree of fire resistance to thehardboard products.

We have now discovered, entirely unexpectedly, that water solublephosphates may be incorporated into lignocellulose hardboard sheets inamounts which greatly im prove the fire resistance of the products. Inits generic aspect, therefore, the invention comprises the addition tolignocellulose hardboard sheets of water soluble ammonium phosphates inamounts sufiicient to render the products fire retardant. Morespecifically, the invention is directed to impregnating hardboardproducts with a mixture of diammonium phosphate and monoammoniumphosphate.

It has been found that lignocellulose hardboard sheets, having densitiesof about 1 or greater, may be first immersed in a hot concentratedaqueous solution of the phosphate mixture, and then immersed immediatelyin a cooler concentrated aqueous solution of the phosphate mixturewhereby the sheets retain, upon drying and/or baking, from about 14% toabout 20% of the phosphate salts based on the dry weight of thehardboard sheets. The treated sheets are characterized by greatlyimproved resistance to fire. The incorporated phosphate salts do notbleed out of the boards, and the products may be decorated with anydesired coating agent such as air dried paint, baked enamel, decorativeplastic sheets, and the treated boards may be laminated or glued by theuse of selected adhesive materials.

The proportions of the two ammonium phosphates to be used are criticalinasmuch as they tend to precipitate from the concentrated solutionsemployed in the novel method unless the limits established are closelyadhered to. Broadly, the range of proportions suitable is from 2:1 tol:2 parts by weight of diammonium phosphate to monoarnrnonium phosphate.Within this operative range, it has been found that optimum results areobtained by 2,766,139 Patented Oct. 9, 1956 employing proportions of 1:1to 1.25:1 parts by weight of diammonium phosphate to monoammoniumphosphate.

The hot bath of concentrated aqueous phosphate mixture is preferablymaintained at about concentration although the amount of salts insolution will obviously vary somewhat as hardboard sheets are immersedtherein. The boiling point of this concentrated solution is about 240 F.and it is preferable to maintain the bath at this temperature althoughthe desired results are obtainable over a bath temperature range ofabout 230- 250 F.

The cooler second bath is preferably maintained at about 60%concentration of phosphate salts although the amount of salts insolution may vary by about 5% above or below the optimum concentration.It has been determined that the second bath should be maintained atabout F. plus or minus 20 degrees.

As stated before, the range of phosphate salts concentration andtemperature ranges are critical inasmuch as the salts precipitate fromsolution where the stated limits are exceeded or fallen short of.

In carrying out the novel method of the invention, the selectedhardboard sheet is first immersed in the hot bath for from about 3minutes for the most dense sheet to about 1 minute in the case of asheet of relatively low specific gravity such as about 1 or slightlyless. Thereafter, the sheet is immediately immersed in the cooler bathfor about 2-0.5 minute depending on the board density. Immersion timesshorter than those specified result in insutficient phosphate saltscontent in the product. Immersion times greater than those specified arenot desirable inasmuch as there is no necessity of exceeding 20%phosphate salts content in the product from the standpoint of fireresistance and, in addition, excessive amounts of salts in the sheetresult in lowering the physical properties thereof. Upon removal of thesheet from the second bath, the solution remaining on the sheet surfacesmay be removed as by washing with water and the impregnated sheet may bedried and/or baked. Uptake of phosphate salts lies within the range ofabout 1420% based on the dry weight of the hardboard sheet.

Where the fire resistant sheet is to be coated with an air drying agent,it is only necessary to dry the sheet to remove water. However, wherethe impregnated sheet is to be finished with a baked enamel or similarlyheat treated agent, it is preferable to bake the sheet to preventgassing during curing of the enamel or other coating. It has been foundthat the baking cycle must be carried out over a progressivelyincreasing temperature range of about -275 F. extending over a period ofabout 1216 hours. Subsequent to this baking treatment the sheet may besubjected to additional heat treatment without adversely affecting thecoating material and without appreciable loss of fire resistance.

While the exact manner in which the hot-cold impregnation methodfunctions is not known, it is believed that immersion of a hardboard inthe hot bath serves to drive out some of the air and moisture present inthe sheet. In this respect, it is important that the hardboard to beimpregnated should have a minimum moisture content of 3.5% andpreferably about 5%. Some impregnation of the hardboard occurs duringthis hot bath immersion. However, when the hot sheet is immersed in thesecond and relatively cool bath, there is apparently a vacuum effect onthe voids present in the sheet. The cooler solution is thus enabled topenetrate the sheet very rapidly and thoroughly. In any event, muchgreater uptake of salts is obtained than by any single impregnationmethod. The result is the provision of hardboard containing sufficientammonium phosphate salts to render the treated product fire retardant.

The invention Will bemore fully described by the following examples. Itshould be borne in mind, however, that the examples are given forillustrative purposes alone, and the invention is to be limited only bythe scope of the appended claims.

EXAMPLE 1 Several pieces of 49 square lignocellulose hardboard wereprepared. The square hardboard sheets had specific gravities of about1.12 and were smooth on both surfaces. In sequence, the boards wereimmersed in two aqueous solutions containing 1:1.25 proportions byWeight of diammonium phosphate and monoammonium phosphate under thefollowing conditions:

Solution 1approximately 80% concentration of phosphate mixture.Temperature 241i1 F. Boards immersed for 3 minutes.

Solution 2-approximately 60:1.5% concentration of phosphate mixture.Temperature l22i9 F. Boards immersed for 2 minutes.

Escape of the evolved gases beneath the boards was permitted by use of atilted hold-down for the boards. After being removed from the secondsolution, the boards were Washed with Water, allowed to drain, and driedemploying a graduated temperature 16 hour baking cycle at 140275 F. Theimpregnated board products had an average specific gravity of 1.28 andcontained an average of 18.3% of ammonium phosphate by weight. Thetreated boards were tested for flame resistance according to FederalSpecification SSA118b of August.4, 1954, paragraphs 4.3.3, 4.3.3.1,4.3.3.3 and 4.3.3.4 All results represent average figures for threetests:

SSA118b Federal fire test (40 minute exposure) Weight loss 34.0%

Initial flame 3 min.28 sec.

Cracked through 5 min. 20 sec.

Pieces falling 8 min. 45 sec.

Char diameter 27 inches.

After flame time None.

After glow time None.

After smoke 2.25 min. Light amount. Rated Fire retardant.

In the above test, the sample is rated as fire retardant where thecharred area does not extend to the edges of a 30 square inch areaexposed to the flame employed.

In the following examples, the proportions of diammonium phosphate andmonoammonium phosphate and impregnating times were varied over a widerange to illustrate the critical limits of the novel method of theinvention.

EXAMPLE 2 The procedure of Example 1 was repeated employing thefollowing conditions:

Solution 1approximately 80% concentration of phosphate mixture. Ratio-2parts diammonium phosphate- 1 part monoammonium phosphate. Temperature255 F. Board of specific gravity 1.04 immersed for 2.5 minutes.

Solution 2-approximately 5 8% concentration of phosphate mixture. Ratioof phosphates same as solution 1. Temperature 127 F. Board immersed for1.5 minutes.

The impregnated board, after drying, had a phosphate salts content of17.25% and a specific gravity of 1.17. The board was rated as fireretardant.

EXAMPLE 3 T he procedure of Example 1 was repeated employing thefollowing conditions:

Solution 1approxirnately 80% concentration of phosphate mixture. Ratio-2parts diammonium phosphate- 1 part monoammonium phosphate. Temperature246 F. Board of specific gravity 1.01 immersed for 1 minute.

Solution 2-approx-imately 54% concentration of phosphate mixture. Ratioof phosphates same as solution 1.

4. Temperature 147 F. Board immersed for 0.5 minute. The impregnatedboard, after drying, had a phosphate salts content of 14.6% and aspecific gravity of 1.17. The board Was rated as fire retardant.

EXAMPLE 4 The procedure of Example 1 was repeated employing thefollowing conditions:

Solution l-approximately concentration of phosphate mixture. Ratio-1part diammonium phosphate-2 parts monoammonium phosphate. Temperature245 F. Board of specific gravity 1.11 immersed for 3 minutes.

Solution 2approximately 58% concentration of phosphate mixture. Ratio ofphosphates same as solution 1. Temperature F. Board immersed for 2minutes.

The impregnated board, after drying, had a phosphate salts content of17.1% and a specific gravity of 1.25. The board was rated as fireretardant.

All of the fire resistant products of the above examples werecharacterized by good appearance and physical properties equal to thoseof untreated hardboard. The products were coated with both air dryingpaint and baked enamels. The coated articles were entirely satisfactoryin all respects.

From the foregoing description of the invention it will be seen that thenovel method provides a desirable means for preparing fire resistanthardboard products. The invention has the outstanding advantage ofproviding a commercially acceptable method heretofore lacking in theprior art. The method is simple, economical, and provides fire resistantproducts which may be coated with any desired coating material eitherfor general use or for use in the tileboard industry where perfection ofthe surface appearance is of prime importance.

We claim:

1. A method of imparting fire resistance to lignocellulose hardboardwhich comprises submerging a hardboard sheet in a hot aqueous solutionof a mixture if diarnmonium phosphate and monoammonium phosphate for aperiod of time Within the range of about 13 minutes, removing thehardboard sheet from the hot solution, immediately submerging the sheetin a cooler aqueous solution of a mixture of the said two phosphates fora period of time within the range of about 0.5-2 minutes, the twoaqueous solutions containing parts by weight of the two phosphateswithin the range of 2:1 to 1:2 of diammonium phosphate to monoammoniumphosphate, thereafter removing the excess solution from the surfaces ofthe hardboard sheet, and subjecting the impregnated sheet to a bakingcycle.

2. A method of imparting fire resistance to lignocellulose hardboardwhich comprises submerging a hardboard sheet in a hot aqueous solutionhaving a concentration of approximately 80% of a mixture of diammoniumphosphate and monoammonium phosphate for a period of time within therange of about 1-3 minutes, removing the hardboard sheet from the hotsolution, immediately submerging the sheet in a cooler aqueous solutioncontaining a concentration of approximately 60% of the said twophosphates for a period of time within the range of about 0.52 minutes,the two aqueous solutions containing parts by weight of the twophosphates within the range of 2:1 to 1:2 of diammonium phosphate tomonoammonium phosphate, thereafter removing the excess solution from thesurfaces of the hardboard sheet, and subjecting the impregnated sheet toa baking cycle.

3. A method of imparting fire resistance to lignocellulose hardboardwhich comprises submerging a hardboard sheet in a hot aqueous solutionhaving a concentration of approximately 80% of a mixture of diammoniumphosphate and monoammonium phosphate for a period of time within therange of about 1-3 minutes, removing the hardboard sheet from the hotsolution, immediately submerging the sheet in a cooler aqueous solutioncontaining a concentration of approximately 60% of the said twophosphates for a period of time within the range of about 0.5-2 minutes,the two aqueous solutions containing parts by weight of the twophosphates within the range of 2:1 to 1:2 of diammonium phosphate tomonoammonium phosphate and the solutions being maintained attemperatures within the ranges of 230250 F. and 100l40 F., respectively,thereafter removing the excess solution from the surfaces of thehardboard sheet, and subjecting the impregnated sheet to a baking cycle.

4. A method of imparting fire resistance to lignocellulose hardboardwhich comprises submerging a hard board sheet in a hot aqueous solutionhaving a concentration of approximately 80% of a mixture of diammoniumphosphate and monoammonium phosphate for a period of time within therange of about 1-3 minutes, removing the hardboard sheet from the hotsolution, immediately submerging the sheet in a cooler aqueous solutioncontaining a concentration of approximately 60% of the said twophosphates for a period of time Within the range of about 0.52 minutes,the two aqueous solutions containing parts by weight of the twophosphates Within the range of 2:1 to 1:2 of diammonium phosphate tomonoammonium phosphate and the solutions being maintained attemperatures within the ranges of 230 250 F. and 100 140 B,respectively, thereafter removing the excess solution from the surfacesof the hardboard sheet, and subjecting the impregnated sheet to a bakingcycle at a graduated increasing temperature range of 140 F.275 F.extending over a period of 12-16 hours.

5. A method of imparting fire resistance to lignocellulose hardboardwhich comprises submerging a hardboard sheet in a hot aqueous solutionof a mixture of diammonium phosphate and monoammonium phosphate for aperiod of time within the range of about 1-3 minutes, removing thehardboard sheet from the hot solution, immediately submerging the sheetin a cooler aqueous solution of a mixture of the said two phosphates fora period of time within the range of about 0.5-2 minutes, the twoaqueous solutions containing parts by weight of the two phosphatesWithin the range of 1:1 to 1.25:1 of diammonium phosphate tomonoammonium phosphate, thereafter removing the excess solution from thesurfaces of the hardboard sheet, and subjecting the impregnated sheet toa baking cycle.

6. A method of impregnating a dense lignocellulose hardboard with fromabout 16% to about 20% by weight of water soluble phosphates whichcomprises submerging the hardboard sheet in a hot aqueous solution of amixture of diammonium phosphate and monoammonium phosphate for about 3minutes, removing the hardboard sheet from the hot solution, immediatelysubmerging the sheet in a cooler aqueous solution of a mixture of thesaid two phosphates for about 2 minutes, the two aqueous solutionscontaining parts by weight of the two phos phates within the range of1:1 to 1.25 :l of diammonium phosphate to monoammonium phosphate atconcentrations of about 80% and in the respective hot and coolsolutions, thereafter removing the excess solution from the surfaces ofthe hardboard sheet, and drying the sheet.

7. A method of imparting fire resistance to lignocellulose hardboardwhich comprises submerging a hardboard sheet for about 3 minutes in anaqueous solution having a concentration of approximately of a mixturecontaining equal parts by weight of diammonium phosphate andmonoammonium phosphate, said solution being maintained at about 240 F.,removing the hardboard sheet from the solution, immediately submergingthe sheet in a second aqueous solution of said phosphates for about 2minutes, said second solution containing similar 1:1 proportions ofdiammonium phosphate and monoammonium phosphate by weight at 60%concentration plus or minus about 5%, said second solution beingmaintained at about F. plus or minus about 5 F., thereafter removing theexcess solution from the surfaces of the hardboard sheet, and subjectingthe impregnated sheet to a baking cycle at a graduated increas ingtemperature range of -275 F. extending over a period of 12-16 hours.

References Cited in the file of this patent UNITED STATES PATENTS1,942,977 Payne Ian. 9, 1934 FOREIGN PATENTS 1,126 Great Britain 18751,187 Great Britain 1856

1. A METHOD OF IMPARTING FIRE RESISTANCE TO LIGNOCELLULOSE HARDBOARD WHICH COMPRISES SUBMERGING A HARDBOARD SHEET IN A HOT AQUEOUS SOLUTION OF A MIXTURE IF DIAMMONIUM PHOSPHATE AND MONOAMMONIUM PHOSPHATE FOR A PERIOD OF TIME WITHIN THE RANGE OF ABOUT 1-3 MINUTES, REMOVING THE HARDBOARD SHEET FROM THE HOT SOLUTION, IMMEDIATELY SUBMERGING THE SHEET IN A COOLER AQUEOUS SOLUTION OF A MIXTURE OF THE SAID TWO PHOSPHATES FOR A PERIOD OF TIME WITHIN THE RANGE OF ABOUT 0.5-2 MINUTES, THE TWO AQUEOUS SOLUTIONS CONTAINING PARTS BY WEIGHT OF THE TWO PHOSPHATES WITHIN THE RANGE OF 2:1 TO 1:2 OF DIAMMONIUM PHOSPHATE TO MONOAMMONIUM PHOSPHATES, THEREAFTER REMOVING THE EXCESS SOLUTION FROM THE SURFACES OF THE HARDBOARD SHEET, AND SUBJECTING THE IMPREGNATED SHEET TO A BAKING CYCLE. 